The invention relates generally to manufacturing processes for tube bending and, more particularly, to laser tube bending.
Presently, mechanical bending techniques are generally used to bend tubes. Although mechanical tube bending systems can bend tubes quickly with acceptable radius control, these systems have a number of limitations. For example, mechanical bending usually requires dedicated fixtures, thereby increasing the expense of the process. Mechanical bending often causes thinning along the outside arc of the bend radius (the extrados), thereby necessitating in many cases the use of a heavier tube than would otherwise be required. In addition, mechanical bending around large radii often causes cross-section distortion, which reduces the cross sectional area of the bent tube (important if the tube is to carry a gas or fluid), as well as reducing rigidity. In addition it is difficult to create contiguous (one after another) or compound (out of plane) bends using mechanical bending techniques. Contiguous bends are difficult because bending dies by their nature often have inherent minimal requirements for straight sections before and after the bend. Out of plane bends are also limited to applications with specialty tooling. These restrictions are typically overcome by cutting segments of bent pipe and bonding to form the desired configuration—steps that add to manufacturing complexity, time and cost. Hydroforming has been applied to tube forming to alleviate some of the drawbacks of traditional mechanical bending, but the advantages are offset by high equipment costs and specialized tooling.
As an alternative to mechanical bending, thermal energy can be used to bend tubes. Thermal tube bending imparts permanent deformations and is a non-contact process that is free from the use of dies and the concomitant constraints of the mechanical countertype. The thermal mechanism employed is typically referred to as an “upsetting” mechanism that contracts or gathers (hence the term “upset”) material at the point of heating. The judicious application of heating over large areas will shape the workpiece in a desired fashion, allowing for example tube sections to be formed into a variety of shapes.
The mechanism of thermal upsetting can be described as follows: consider a thin plate or sheet of material that expands upon heating, as do most engineering materials such as metals, thermoplastics, etc. If a heat source is applied to a region of the plate such that a small temperature gradient perpendicular to the surface is produced—i.e. the temperature is nearly constant through the thickness—a compressive stress will begin to develop in the plane of the sheet—given that buckling is not introduced—due to the thermal expansion of the heated material. At first, only elastic deformations are present, and if the thermal excursion is gentle enough, upon cooling, the plate will return to its original planar shape. If however the thermal energy continues to be applied—baring melting—the in-plane compressive stresses will yield the material and it will begin to plastically flow in compression in the plane of the plate. Because the through-thickness thermal gradient is very low, the plastic flow will be nearly homogenous in the thickness direction. During cooling the heated region will contract, and after complete cooling the region will exhibit a net contraction in the plane orientation and a net expansion in the thickness orientation due to the plastic flow. The mechanism can be effectively applied with moving heat sources (resulting in a predominant contraction transverse to the heating line) with various geometries. Any of a number of thermal energy sources may be used such as laser, induction, resistance, plasma, etc.
In laser tube bending, laser energy is usually scanned across the inside arc of the intended bend (the intrados) to heat the tube. The tube then is allowed to cool. The thermal stress causes plastic thickening and contraction of the scanned region as described above, while the opposing side maintains its original length, thereby causing the tube to bend toward the scanned region. Under favorable conditions, laser tube bending maintains both the outside arc thickness and tube cross section. Laser tube bending also minimizes the need for hard tooling, which can reduce costs and lead times significantly and permits users to create complex combinations or configurations of bends including out-of-plane or three-dimensional (3D) bends that would otherwise be prohibitively expensive to make by traditional methods.
Laser tube bending can be performed using either a rotational or an axial application of energy. For axial scanning, a specially shaped laser beam is directed along the axis of the tube. Axial scanning is generally suitable for large radius tube bending. For the rotational approach, laser energy is applied in the tube's circumferential direction at discrete intervals.
In addition, the rotational approach may be used for large radius tube bending but may be slower than axial tube bending. Usually, axial scanning is faster than rotational scanning. However, with axial scanning it can be difficult to control surface damage, in addition to being difficult to achieve small bending radii. Axial scanning is also particularly sensitive to the processing window and cooling of the tube. Beneficially, rotational tube bending provides improved, localized control of the bending, thereby providing improved accuracy. However, current rotational tube bending methods are quite slow.
It would therefore be desirable to provide a system and method for laser tube bending that have enhanced robustness and greater speed relative to existing systems and methods.